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Spatial Distribution of Foliar Diseases in Soybeans

Mckayla Patterson: University of Arkansas-Fayetteville

<div>Asian soybean rust, caused by <em>Phakopsora pachyrhizi, </em>Cercospora leaf blight, caused by the fungus<em> Cercospora kikuchii, </em>and Frogeye leaf spot, caused by the fungus<em> Cercospora sojina </em>were identified in a field near Hamburg, AR. Fifty GPS points were marked at approximately equal distances and foliar disease levels were rated in the top 1/3 of the soybean canopy. At each point, 10 leaflets were collected and imaged using a flatbed scanner. Leaflet images were analyzed for percent defoliation of each of the three diseases present by point. Spatial analysis of disease levels were conducted to determine distributions using Moran’s I in Geoda. Asian soybean rust and Cercospora leaf blight were significantly clustered (P=0.05) while Frogeye leaf spot occurred at very low levels and was randomly distributed throughout the field (<em>NS</em>). When distributions of Asian soybean rust and Cercospora leaf blight were compared, their distributions were significantly dispersed (P=0.05) indicating some level of biological competition. These results are significant as foliar diseases are typically considered to be randomly distributed in fields. However, the clustered natures of the diseases found in this location are in disagreement with this idea. These results point to opportunities for predictive scouting or preferential fungicide trial placement in the future.</div>